Various types of optical, scattering, smoke detectors are known. They provide useful warnings of developing smoke conditions. One such structure is disclosed in U.S. Pat. No. 6,521,907 (“the '907 patent”), entitled “Miniature Photoelectric Sensing Chamber”, which issued Feb. 18, 2003 and is assigned to the assignee hereof. The '907 patent is incorporated by reference herein.
In summary, such optical smoke detectors or multi-criteria smoke detectors that use an optical signal to detect fire include a sensing chamber where smoke enters an optical system to detect light scattered by smoke particulate, possibly other transducers (thermistors, etc.), electronic control circuits, and a communication system to process signals from the transducers. Information from a detector can be transmitted to a fire alarm control panel. However, some types of detectors do not communicate with a control panel, but have an integrated alarm system.
In known smoke and fire detectors, the optical system includes an optical emitter and a receiver that are integrated with the sensing chamber of the detector through a use of an optic part holder. Among other functions, this part holder facilitates automatic assembly of the detector.
The optical system has to meet various needs and requirements to be suitable for its purpose. Known needs and requirements can include acceptable optical sensitivity to guarantee a good signal to noise ratio in a presence of smoke, immunity to small non-smoke particulate matter or bugs that enter the sensing chamber, and immunity to condensation and humidity.
Small size due to reduced chamber volume is an asset as is an ability to cost effectively assemble such detectors using automatic placement machines.
As those of skill will understand, the optical emitter and the receiver have to be located so that, without smoke, only a very little amount of light reaches the receiver after multiple reflections in the sensing chamber. On the other hand, in the presence of smoke, a sufficient amount of the light projected by the optical emitter is scattered by smoke particles and collected by the receiver so that the presence of smoke can be evaluated.
It has also been recognized that a variety of interfering phenomena can adversely impact a performance of such devices. These include dust, insects, or small objects that can enter the sensing chamber and cause a signal drift or false alarms. High humidity or condensation phenomena in the sensing chamber can also effect unwanted signal variations.
Different configurations of the optical system in commercial fire detectors are known. The optical emitter and the receiver can be soldered to a printed circuit board. An optical set-up is assured through the use of one or more molded optic part holders. The optic part holder can also reduce a beam of the light from the optical emitter in order to get a larger optical signal only in the presence of the smoke in the sensing chamber.